U.S. Pat. No. 8,792,087 B2 to Andreas Spickerman et al. discloses a device in which at least two different transfer gates which couple a photoactive region to at least two different evaluating capacities are driven during different drive intervals so that charge carriers generated during the drive intervals by a radiation pulse reflected from the object of measurement and/or by ambient radiation can be transported from the photoactive region to the evaluating capacities each coupled to the at least two transfer gates. Another transfer gate is driven during a time outside the drive intervals of the at least two transfer gates to connect the photoactive region to a reference potential terminal acting as a charge carrier sink during the time outside the drive intervals of the at least two transfer gates.
It is a disadvantage of the system known from U.S. Pat. No. 8,702,087 B2 that the pixels get saturated by reflections from objects at short distances from the sensor.
United States patent application publication no. US 2007/158770 A1 to Shoji Kawahito, discloses a range-finding image sensor based upon measurement of reflection time of light with reduced fabrication processes compared to standard CMOS manufacturing procedures. An oxide film is formed on a silicon substrate, and two photo-gate electrodes for charge-transfer are provided on the oxide film. Floating diffusion layers are used to convert charges to electronic potential, a mechanism traditionally inherited from the legacy technology of Charged Coupled Devices (CCD). Extra transistors are provided for resetting and a diffusion layer to provide a given reset voltage.
It is a disadvantage of the pixel disclosed in US 2007/158770 A1 that it uses non-standard technology and that the pixel design does not allow the addition of additional wells without sacrificing active surface area of the pixel. This is suboptimal for usage in sensor systems with ultra low power lasers requiring large operational range. The used process is not commonly available in standard CMOS processes, which reduces this concept's applicability and its ability to be produced at an affordable cost in large volumes.
The range of a sensor based on such a design is also limited at the near end by saturation of the pixels by the strong reflections of projected light.
The saturation of pixels when sensing short-range reflections, or highly reflective objects such as traffic signs, license plates, etc., is especially problematic when the pixels are used in sensors for automotive applications, as is the purpose of the pixel according to the present invention, because Advanced Driver Assistance Systems (ADAS) and self-driving cars require high accuracy at short range. Moreover, in this application domain, accuracy at longer ranges, the ability to operate in bright ambient light conditions, and the requirement of compactness (requiring the use of solid-state semiconductor components) must not be sacrificed for the requirement of short-range accuracy.
It is therefore an object of embodiments of the present invention to overcome the short-range saturation problem for pixels used in range-gating based imaging systems by proposing a different pixel architecture.
Unpublished European patent application no. EP15191288.8, dated 23 Oct. 2015, in the name of the present applicant, describes a system for determining a distance to an object comprising: a solid-state light source arranged for projecting a pattern of spots of laser light towards the object in a sequence of pulses; a detector comprising a plurality of picture elements, the detector being configured for detecting light representing the pattern of spots as reflected by the object in synchronization with said sequence of pulses; and processing means configured to calculate the distance to the object as a function of exposure values generated by said picture elements in response to said detected light; wherein the picture elements are configured to generate said exposure values by accumulating, for each pulse of said sequence, a first amount of electrical charge representative of a first amount of light reflected by said object during a first predetermined time window and a second electrical charge representative of a second amount of light reflected by said object during a second predetermined time window, said second predetermined time window occurring after said first predetermined time window. Each of the plurality of picture elements may comprise at least two charge storage wells, and the detecting of the first amount of light and said detecting of said second amount of light occurs at respective ones of said at least two charge storage wells. EP15191288.8 does not describe a solution to the short-range saturation problem.
In a similar system, unpublished European patent application no. EP16192105.1, dated 3 Oct. 2016, in the name of the present applicant, describes that for given total pixel space, the saturation problem can be mitigated by using an asymmetric well arrangement, in which the photon capacity represented by the first well is increased, and the photon capacity represented by the second well is decreased. If the increase and decrease are balanced, an increase of the dynamic range can be obtained at no additional pixel surface cost, thus maintaining the same resolution. EP16192105.1 does not describe any specific ratios between the photon capacity represented by the first well and the photon capacity represented by the second well.